1. Field of the Invention
The present invention relates to a vehicle motion control apparatus, particularly relates to the vehicle motion control apparatus for regulating a hydraulic braking pressure supplied to at least one wheel brake cylinder out of a pair of wheel brake cylinders included in a hydraulic circuit, to restrain an excessive oversteer and/or an excessive understeer, thereby to maintain stability of a vehicle in motion.
2. Description of the Related Arts
As for a vehicle motion control apparatus, there is disclosed in Japanese Patent No. 3058172, which corresponds to the U.S. Pat. No. 4,898,431, for example, an apparatus for controlling vehicle motion, by determining a desired yaw rate of a vehicle, and controlling braking force in response to a comparison of the desired yaw rate with a sensed actual yaw rate of the vehicle to maintain a vehicle stability during the vehicle motion.
In Japanese Patent Laid-open publication No. 10-211873, a vehicle posture control apparatus has been proposed, so as to enable a vehicle driver to depress a brake pedal even when a vehicle posture control is being performed, and enable the posture control to be performed by his own braking operation. In this publication, it is described that the apparatus is provided with two circuits of brake conduits for communicating a master cylinder with a pair of brake cylinders out of four brake cylinders, respectively, a pair of cut-off valves for shutting off the communication between the master cylinder and the brake conduit of each circuit to be capable of being opened or closed, and a hydraulic pressure source provided for supplying brake pressure individually to the two circuits of brake conduits. As for means for controlling the vehicle posture, there are disclosed a main control section for regulating the brake pressure supplied from the pressure source into each brake cylinder, with the pair of cut-off valves placed in their closed positions to shut off the communication between the master cylinder and all of the brake cylinders, and an opening motion control section for controlling one of the cut-off valves to be changed over to an open state, when braking operation by the vehicle driver is detected by braking operation detection means.
Also, in Japanese Patent (PCT) Laid-open publication No. 2000-503279, which corresponds to the U.S. Pat. No. 6,074,018, there is disclosed a motor vehicle braking system with traction control and/or movement dynamics regulating device, as explained hereinafter. That is, to enable building up brake pressure rapidly, one additional pump is provided in each brake circuit, whose intake side is connected directly to a master cylinder, and between an intake side of the additional pump and the master cylinder, there are no hydraulic components that would act as throttles and thereby delay the brake pressure buildup. Then, with respect to an improvement of the intake side of the additional pump and its effect, it is described that the invention has the advantage of a rapid brake pressure buildup when the master cylinder is not actuated. Furthermore, it is described (with reference numerals omitted herein) that a switchover valve is embodied as a controllable differential pressure valve, that is, a pressure difference between the wheel brake cylinder side and the master cylinder side can be established, with the pressure higher on the wheel brake cylinder side. In the exemplary embodiment shown, the switchover valve is a differential pressure proportional magnet valve. Then, it is described that if a tendency to locking or slipping occurs at one of the vehicle wheels, or if the electronic control unit by means of its gyroscope finds that the vehicle is threatening to skid, then the pump motor is turned on, and that an individual-wheel brake pressure regulation is effected in a manner known per se using the return pump, brake pressure buildup valves, and brake pressure reduction valves.
In Japanese Patent Laid-open publication No. 11-301435, there is disclosed a linear pressure difference valve for use in a brake-by-wire system, similar to the valve as shown in FIG. 1 of the publication No. 2000-503279. Then, it is described (with reference numerals omitted herein) that the linear pressure difference valves allow the flow of brake fluid between a reservoir and each wheel cylinder, almost without flow resistance, at a communication position through each valve member. A valve state with the valve member placed in a pressure difference position is controlled by electric current fed to each solenoid, to control an amount of the valve member lifted from a valve seat. In the pressure difference position, the state of the valve member can be controlled from a position for completely shutting off a conduit to a throttle position, in response to the lifted amount. In the case where the lifted amount corresponds to an approximately intermediate position from the valve seat, the throttle position has been provided for the conduit, thereby to restrict the brake fluid from flowing from the wheel cylinder to the reservoir. As a result, the brake pressure in the wheel cylinder (wheel cylinder pressure) can be held with the pressure difference against the reservoir. And, there is disclosed in Japanese Patent Laid-open publication No. 9-240455, which corresponds to the U.S. Pat. No. 6,142,581, a reservoir having a function for shutting off an intake passage for a hydraulic pressure pump when introducing brake fluid, as indicated by “200” in FIG. 6 of the Japanese publication No. 9-240455.
In order to improve a decelerating response of a vehicle, when a vehicle driver makes a braking operation while a vehicle motion control is being performed, a vehicle motion control apparatus has been proposed in Japanese Patent Laid-open publication No. 10-24821, wherein braking force applied to each wheel of the vehicle is detected by wheel braking force detection means, and wherein motion control means prohibits a wheel to be controlled from being controlled thereby, when the braking force applied to a wheel not to be controlled by the motion control means exceeds the braking force applied to the wheel to be controlled, while the vehicle motion control is being performed. And, the motion control means includes turning control means for applying the braking force to a first wheel out of all the wheels, so as to modify a vehicle moment for forcing the vehicle to be in a stable state, and deceleration control means for applying the braking force to a second wheel out of all the wheels, except for the first wheel, so as to reduce the vehicle speed.
As for the wheel braking force detection means, in Japanese Patent Laid-open publication No. 10-24821, for example, employed is wheel deceleration detection means for calculating a deceleration of each wheel on the basis of a wheel speed detected by a wheel speed sensor. Therefore, it is described that such an expensive sensor as a master cylinder pressure sensor, or wheel cylinder pressure sensor is not required. More practically, the deceleration control is prohibited, when a brake switch (stop switch) has been turned on for a period longer than a predetermined time, and when acceleration of the wheel not to be controlled is smaller than acceleration of the wheel whose deceleration is to be controlled. Furthermore, in Japanese Patent No. 3045057, disclosed is a vehicle behavior control apparatus for terminating a behavior control when hunting might be caused in the behavior control. In practice, when the braking force is applied to a wheel and released from the same, repeatedly and continuously, it is determined that the hunting might be caused in the behavior control. In order to stable the behavior, therefore, it is proposed to prohibit the braking force from being applied to each wheel.
According to the apparatuses as disclosed in the above publication Nos. 10-211873 and 2000-503279, however, it is required that the hydraulic pressure discharged from the hydraulic pressure pump is regulated by controlling the brake pressure buildup valves and brake pressure reduction valves, with the communication with the master cylinder being shut off, when the control for maintaining a stability of the vehicle in motion (i.e., vehicle stability control) is performed. Therefore, required is a master cylinder pressure sensor as disclosed in the above publication No. 10-211873 for detecting the master cylinder pressure discharged in response to braking operation of the vehicle driver while the vehicle stability control is being performed. And, presumably, the apparatus as disclosed in the above publication No. 2000-503279 will also require a hydraulic pressure sensor similar to the sensor as described above.
However, the pressure sensor for detecting the master cylinder pressure is very expensive, so that if it is not required to detect the operation of the brake pedal during the vehicle stability control, and if the hydraulic pressure sensor can be omitted, a great cost down can be achieved. Supposing that the linear pressure difference valve as disclosed in the above publication No. 11-301435, or known linear solenoid valves are employed, and that switching valves for use in the vehicle stability control are controlled in a different manner from the prior control manner, the vehicle stability control can be performed smoothly, without the expensive master cylinder pressure sensor provided in the apparatus.
As for an embodiment for controlling valves or the like in the stability control, may be employed for example a so-called diagonal control system, wherein a hydraulic circuit connected to wheel brake cylinders is divided into two hydraulic circuits each including a pair of wheel brake cylinders, and a hydraulic pressure regulating device is disposed between a master cylinder and a pair of wheel brake cylinders included in each hydraulic circuit, and wherein one of the pair of wheel brake cylinders is selected to be the one operatively associated with the wheel to be controlled, so that the hydraulic braking pressure therein is regulated in response to the vehicle state variable, whereby the vehicle stability control may be performed smoothly, without the master cylinder pressure sensor. In the case where the vehicle stability control is being performed according to the diagonal control system, however, when the brake pedal is depressed to terminate the vehicle stability control, the hunting might be caused, so that the stability control might not be terminated smoothly.
In the mean time, it is described in the above publication No. 10-24821 that without using any expensive sensors such as master cylinder pressure sensor or wheel cylinder pressure sensor, motion control means can prohibit the wheel to be controlled from being controlled, when the braking force applied to the wheel not to be controlled exceeds the braking force applied to the wheel to be controlled. However, this relates to the deceleration control on the basis of the comparison between the braking force applied to the wheel to be controlled and the braking force applied to the wheel not to be controlled, without directly using the brake pressure increased in response to operation of the brake pedal.
Although it is proposed in the Japanese Patent No. 3045057 that when the hunting might be caused in the behavior control, the behavior control shall be terminated. As a result, the hunting will be prevented, while a trace property will be deteriorated. It is, therefore desired that the hunting can be prevented by decelerating the vehicle speed, with the trace property being maintained as the effect of the stability control. Particularly, in case of the diagonal control system as described above, the apparatus as disclosed in the Japanese Patent No. 3045057 will not act as an appropriate countermeasure against the hunting that will be caused when the brake pedal is depressed during the stability control, thereby to terminate the control.